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Marketable Securities (Tables)
6 Months Ended
Jun. 30, 2014
Investments, Debt and Equity Securities [Abstract]  
Investments in marketable securities
Our investments in marketable securities consisted of the following:
 
As of June 30, 2014
 
As of December 31, 2013
(millions)
Amortized
Cost
 
Fair
Value
 
Amortized
Cost
 
Fair
Value
Corporate debt securities
$
92

 
$
92

 
$
87

 
$
87

U.S. government and agency debt securities
4

 
4

 
12

 
12

Asset-backed debt securities
17

 
17

 
20

 
20

Certificates of deposit
19

 
19

 
17

 
17

Municipal debt securities
6

 
6

 
6

 
6

Total marketable securities
$
138

 
$
138

 
$
142

 
$
142

Contractual maturities of marketable securities
Contractual maturities of marketable securities as of June 30, 2014 were as follows:
(millions)
Amortized
Cost
 
Fair
Value
Due in 1 year or less
$
79

 
$
79

Due in 1-5 years
59

 
59

Total marketable securities
$
138

 
$
138